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Yoshiki Tanabe, Mayu Onodera, Masato Nakatsukasa, Yutaka Kunimatsu, Hi ...2020 Volume 126 Issue 4 Pages 167-181
Published: April 15, 2020
Released on J-STAGE: July 31, 2020
JOURNAL FREE ACCESSCane rats are represented by two extant species inhabiting Sub-Saharan Africa. Previous studies have established that the earliest occurrence of the extant genus Thryonomys was the latest Miocene in eastern Africa. A Japan-Kenya Expedition team has recovered a new species of Thryonomys, T. kamulai, sp. nov., from the base of the upper Miocene Nakali Formation (~10 Ma) of northern Kenya. We describe the new species on the basis of dental specimens. The new species is characterized by features such as small dentition size, shallower and wider sinuses on the upper cheek teeth, and a well-developed crescent-shaped metalophulid I on dp4. The present findings extend the first-appearance datum of the genus to the early late Miocene (~10 Ma), suggesting that the Thryonomys lineage inhabited Africa prior to ~10 Ma.
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Tomonori Naya, Kiyohide Mizuno2020 Volume 126 Issue 4 Pages 183-204
Published: April 15, 2020
Released on J-STAGE: July 31, 2020
JOURNAL FREE ACCESS
Supplementary materialWe investigated the stratigraphy of the Lower Pleistocene Bushi Formation in the Kaji Hills, western Kanto Plain, central Japan. A total of 27 tephra layers, including some widespread tephras, are distinguished. The Bushi Formation is divided into five lithological units (A to E) on the basis of the lithofacies characteristics of depositional cycles in each unit. A total of eight marine beds (M1 to M8) are recognized from lithofacies and diatom assemblages. The depositional age of each unit is estimated by correlation of widespread tephras and marine beds, as follows: Unit A: 2.5-2.4 Ma, Unit B: 2.4-2.2 Ma, Unit C: ~1.8 Ma, Unit D: 1.7-1.6 Ma, and Unit E: 1.6-1.4 Ma. Alternating layers of shallow-marine and fluvial deposits in the interval between the Azu-Koen4 tephra layer and the M5 marine bed can be considered as a depositional cycle that represents eustatic sea-level change during marine oxygen isotope stages 93 to 89.
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Kazuhiko Kano, Osamu Ishizuka, Shun Nakano, Takeshi Ohguchi2020 Volume 126 Issue 4 Pages 205-213
Published: April 15, 2020
Released on J-STAGE: July 31, 2020
JOURNAL FREE ACCESSTatsuko-Tai and Shinko-Tai are lava domes located under Lake Tazawa（Tazawa-Ko or Tazawako in Japanese）, Akita Prefecture, NE Japan as inferred from observed high magnetic anomalies and dacite and andesite (or dacite) samples, respectively, collected from the domal masses. Recently obtained acoustic sounding data and underwater video images were used to characterize the surface morphology. Shinko-Tai consists of a symmetric lava dome and a flow of lava branching westwards from the dome, whereas Tatsuko-Tai is a slightly asymmetric and smaller single lava dome that shows coarse columnar joints on its surface. The dacite sample collected from Tatsuko-Tai yields a 40Ar/39Ar plateau age of 1.73±0.08 (1σ) Ma. This age is consistent with the ages of lava flows along the ridge of the caldera rim, suggesting that the Tazawako caldera formed earlier than 1.7 Ma.
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Tsuyoshi Sazawa, Julien Legrand, Atsushi Yabe, Sachiko Agematsu, Katsu ...2020 Volume 126 Issue 4 Pages 215-221
Published: April 15, 2020
Released on J-STAGE: July 31, 2020
JOURNAL FREE ACCESSA palynostratigraphic study was conducted on the Upper Cretaceous Asuwa Formation cropping out at Ikeda Town, eastern Fukui Prefecture, central Japan, to make a precise determination of its age. An abundant and diverse assemblage of palynomorphs (52 genera and 72 species) was obtained from three horizons of mudstone in the upper member (the Sarao Interbedded Sandstone and Mudstone Member). A common stratigraphic range of index angiosperm pollen belonging to the Triprojectate (5 genera and 8 species) and Oculata (1 genus and 2 species) types indicates a Maastrichtian age for the deposit. This age is concordant with that of the Omodani Rhyolite, which is located ~40 km to the east of Ikeda, and those of later stages of the Nohi Rhyolites. The results provide important constraints on the timing and western expansion of rhyolitic volcanic activity 50 km toward the Sea of Japan in western-central Japan.
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Yusuke Shimura, Makoto Takeuchi, Tetsuya Tokiwa, Akari Ota2020 Volume 126 Issue 4 Pages 223-230
Published: April 15, 2020
Released on J-STAGE: July 31, 2020
JOURNAL FREE ACCESS
Supplementary materialThe depositional age sequence of Cretaceous accretionary complexes in the Kouyasan Belt of the Shimanto Belt (central Kii Peninsula, SW Japan) is poorly constrained owing to a lack of age data for the Unokawa Complex in this belt. We conducted U-Pb dating of detrital zircons from three sandstone samples of the Unokawa Complex. Detrital zircon U-Pb ages in this complex consist mainly of two age groups at ca. 300-100 Ma and ca. 2600-1700 Ma. The youngest single grain ages are 110-102 Ma, and the youngest cluster ages are 124-102 Ma, ranging from the Aptian to Albian. These youngest ages of the Unokawa Complex overlap (at 2σ) with those of the underlying Takaharagawa Complex but do not overlap with those of the overlying Mugitani Complex (Mg2 Unit).
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